The present invention relates to the correction of calibration errors, particularly for optical instruments.
Optical instruments generally comprise an arrangement of optical elements such as lenses of a telescope or camera for viewing or to focus onto an object. Further, optical instruments such as a tachymeter or a leveling device are available allowing determining a position of an object relative to a position of the optical instrument. A leveling device generally indicates the relative position of a target object based on a vertical direction only, i.e. the elevation of the target object relative to the position of the leveling device. A tachymeter, on the other hand, may indicate the position of a target with a vertical component and a horizontal component.
The position of the object may be indicated by angles specifying a deviation of a line connecting the object and the instrument from e.g. a known horizontal and/or a known vertical direction. Often, optical instruments indicate the relative position of an object by using spherical coordinates. For example, the position of an object may be defined by Cartesian coordinates defined with respect to a Cartesian coordinate system having three axes orthogonal to each other with the instrument located at the origin. For measuring positions, spherical coordinates, however, are more appropriate. The position of the object relative to the instrument may thus be defined in spherical coordinates by its distance to the origin of the orthogonal coordinate system, an angle between one of the horizontal axes of the coordinate system and a line connecting the origin of the coordinate system with a projection of the point onto the horizontal plane and finally a vertical angle between the coordinate system axis orthogonal to the horizontal plane and a line connecting the origin of the coordinate system and the point. Cartesian coordinates can be transformed into spherical coordinates and vice versa.
In all cases very precise measurements of such angles are required. Conceivably, a relatively small error in a measured angle may translate into a potentially large positioning error that may be unacceptable for example in construction applications, especially if the position of an object in a larger distance to the optical instrument is be determined.
To reduce the occurrence of measurement errors an optical instrument such as a tachymeter or leveling device usually is adjusted or calibrated. A calibration generally is performed before use of the instrument and/or upon manufacture. With the calibration measurement errors of the instrument are quantified and used for correcting an actual measurement, e.g. of a position of a target object. For example, if it is known from a test measurement that an optical instrument determines a viewing direction with a 1 degree error, an actual measurement can be corrected by compensating for the 1 degree error, i.e., adding or subtracting 1 degree from the measurement result depending on the direction of the 1 degree measurement error. The determined measurement error can be used to set the initial adjustment of the optical instrument, such as an indicator of an angle or used to correct an angle indicated by the instrument.
While the above outlined calibration of optical instruments works well for many applications, error sensitive applications may require an error correction with higher accuracy. Moreover, applications requiring a large number of consecutive measurements may require repeated calibration steps for calibrating the optical instrument, thus reducing productivity.